You can look athttp://en.wikipedia.org/wiki/Heavy_water#Effect_on_biological_systems
Basically, bonds involving deuterium can be of a different strength than bonds with hydrogen-1, and since vibrations are slower with a heavier atom, chemical reactions are slowed down. This messes up some biochemical processes which are very fine tuned to work with normal water. For bacteria, they are slowed down but are still able to live and reproduce. In mammals, a large change in the concentration of D20 is fatal. It would take a very high amount of heavy water to kill someone, around a quarter of the weight of their entire body, but it is possible in principle.